The invention relates to a device for the ultrasonic measuring of cylindrical test specimen, in particular tubes and hoses, wherein this device can be filled with water or submerged in a water bath and is provided with at least one ultrasonic measuring head. The invention furthermore relates a corresponding method.
Tubes and hoses made from plastic or metal are normally produced with the extrusion method, for which the plastic or the metal is pressed through an extrusion die and/or nozzle that determines the dimensions of the extruded product. Different factors such as temperature, pressure, mass flow, discharge speed and setting errors of the extruding equipment, just to name a few, influence the quality of the end product and can, for example, lead to deviations in the cross-sectional geometry.
Various methods are known already for measuring the extruded products, either in-line or with the aid of test specimen in the test laboratory where mechanical micrometers and optical measuring devices are mainly used.
Profile projectors are frequently used with the optical measuring instruments, which project the shadow of the cross-sectional pattern to be measured onto a screen where it can be measured.
Reference DE 33 03 637 A1 discloses a device for measuring the cross-sectional dimensions of hollow-cylindrical work pieces. For this, a work piece test specimen to be measured is centered relative to three end positions that are arranged in a triangle with uniform legs on the test specimen wall and two diametrically opposite arranged measuring systems are arranged on the center perpendicular line for the connecting line between the two other end stops on the outside wall of the test specimen. The end position on the inside in this case is a rotating, cylindrical holding mandrel with a defined diameter and a horizontal center axis, wherein the inside wall of the test specimen rests in the manner of a line on this holding mandrel.
With this known device, the preparations for obtaining the desired measuring results are very involved and measuring errors can also occur easily.
Furthermore known is a method for determining the wall thickness of tubes and hoses and/or hollow cylindrical work pieces in the production line or the test laboratory with the aid of ultrasonic measuring. Thus, a printed article by the Danish Atomic Energy Commission, entitled “Tube Inspection System,” describes a measuring device for tubes where the wall thickness of a tube is measured from the outside only with the aid of ultrasound and two opposite arranged, non-contacting sensors.
However, the measuring with the aid of ultrasound was not precise enough for laboratory measurements, primarily because of the direct dependence of the measurement on the sonic speed of the material and its temperature dependence.
A device and a method for determining the wall thickness and sonic speed with the aid of reflected ultrasonic pulses is known from U.S. Patent Application No. 2002/0134159 A1. The content of this published patent application mostly coincides with the explanations provided in the scientific article published by Ping He “Simultaneous measurement of sound velocity and wall thickness of a tube,” in: Ultrasonics 39 (2001), 407–411, Elsevier Science B.V. The device described therein is shown in FIG. 1, which will be explained in further detail later on. The article suggests using the distance between the two ultrasonic measuring heads used as reference value. However, it has turned out that this reference is relatively unstable, wherein the opposite-arranged measuring head surfaces appear to be responsible for the unstable reflections.
As in the latter case, the ultrasonic measurement according to our invention is carried out with water as the coupling medium. The test specimen is submerged for this purpose in a water bath, in which at least some parts of the measuring device, e.g. the ultrasonic measuring heads, are also located. With an ultrasonic measuring in line, the tube to be measured is normally pulled through the water bath.